1. Field of the Invention
This invention relates to a method of enhancing the accuracy of sound velocity measurements made in organic tissue using ultrasound means. More particularly, this invention relates to a method of estimating the change in velocity of ultrasound energy as its passes through multiple tissue layers and is refracted thereby.
2. Background Art
Ultrasound techniques have been extensively used in the field of diagnostic medicine as a non-toxic means of analyzing the properties of in vivo (i.e. living) tissue. Ultrasound can be used for both tissue imaging as well as the measurement of the speed of sound in organic tissue. In particular, the speed of sound in an organ can be one indication of the presence of disease within that organ.
A human or animal body represents a non-homogeneous medium for the propagation of ultrasound energy. The internal area of the body is comprised of a plurality of regions representing different organs, tissue layers, and bone. The speed of sound within each of these layers is, in general, different from that of neighboring regions. As an ultrasound beam passes through the various regions internal to the body, these differing speeds of sound result in the refraction of the ultrasound beam. The beam will, in general, undergo bending at the interface between two such regions. Problems with compensating for refraction have limited the accuracy and effectivenss of ultrasound techniques in the medical diagnostic field.
One principal source of refraction is the layer of fat contained in the tissue wall surrounding the body. This layer can be up to several centimeters thick and will generally have a speed of sound different from that of the internal organs beneath the body fat layer. Also, the inner boundary of the body fat layer may be somewhat irregular. As the ultrasound beam is directed from a source transducer on the surface of the body skin through the fat layer and into internal organs, there is a refraction or bending of the beam as it passes through the inner boundary of the body fat layer.
For some ultrasound techniques utilizing intersecting ultrasound beams, refraction can be a source of inaccuracy. Inability to determine precisely how the various ultrasound beams will be bent may make actual intersection of the ultrasound beams difficult. In appropriate circumstances, the effects of refraction may simply be ignored. In other instances, a reasonable assumption can be made that parallel ultrasound beams are subject to equal bending due to refractive effects.
In many cases, however, a higher degree of accuracy may be obtained by estimating the extent to which ultrasound beams are bent due to significant refractors such as the inner boundary of the body wall. Additionally, there are circumstances under which introduction of refraction with acoustical contrast fluids can be used to enhance the accuracy of in vivo ultrasound examination.
The various effects of refraction on the accuracy of ultrasound examination of organic tissue are complex and frequently strongly interact with one another. Consequently, a single approach to the analysis of and correction for refraction is insufficient in the general case. The medical ultrasound researcher requires a battery of techniques, the techniques useable individually or in combination, to adequately deal with the almost infinite variety of refraction problems encountered in medical ultrasound analysis and diagnosis.